Spatial and temporal variability of phosphorus retention in a riparian forest soil

Riparian zones remove P from surface runoff and can act as filters of nonpoint source (NPS) P pollution for surface waters. Riparian forest soils were investigated in spring and fall for their capacity to retain PO43--P. Samples (300 on each date) were taken from a soil drainage catena from moderately well (MWD), somewhat poorly (SPD), and poorly drained (PD) soil in May and November of 1995 to examine spatial and temporal variability of P retention and its relationship to soil properties. The equilibrium P concentration at zero sorption (EPC0) was determined for each sampling point (lower EPC0, = higher P retention capacity). Mean (coefficient of variation, CV) EPC0 values were 3.8 (0.49) mg P L-1 in May and no P sorption was apparent in November for SPD soil, 1.0 (1.34) and 1.5 (1.03) mg P L-1 in May and November, respectively for MWD soil, and 0.5 (1.87) and 1.3 (1.13) mg P L-1, respectively for PD soil. The EPC0 was significantly and positively correlated to organic matter (OM) for all drainage classes on both dates. Low EPC0 values - high P retention capacity - in MWD and PD soil corresponded with high Fe-ox and Al-ox values. For SPD soil, high mean EPC0 corresponded with low mean Fe-ox and Al-ox values. The relationship between EPC0 and Fe-ox and Al-ox was described by a hyperbolic function for MWD and PD soil, but not for SPD soil. The EPC0 did not exhibit spatial structure at the sampling scale used for any of the drainage classes, even though Fe-ox, Al-ox, and OM content showed spatial structure.